Remove sensitivity/response; fixes issue #117.

Remove instrument_sensitivity if it exist. If it doesn't exist, then
an instrument_polynomial must describe the response, which is how the
Geomagnetism Program specifies an offset and gain response. We do not
attempt to remove any frequency-depenent response since these all seem
tailored to seismic data (e.g., no DC/zero-frequency response).

geomagio/edge/FDSNFactory.py

@@ -258,6 +258,25 @@ class FDSNFactory(TimeseriesFactory):
             data = self._rotate_and_return_requested_channel(
                 data, sncl, starttime, endtime, channel
             )
+
+        for trace in data:
+            response = trace.stats.response
+            if response.instrument_sensitivity:
+                if response.instrument_sensitivity.input_units == "T":
+                    # some ASL stations produce data in Teslas, not nanoteslas
+                    response.instrument_sensitivity.value /= 1e9
+                    response.instrument_sensitivity.input_units = "nT"
+                # apply total gain/sensitivity, ignore any frequency response
+                trace.remove_sensitivity()
+            else:
+                # according to fdsn docs, if no instrument_sensitivity,
+                # the response must be an instrument_polynomial
+                # https://docs.fdsn.org/projects/stationxml/en/latest/reference.html#instrumentsensitivity
+
+                # Geomag Program observatories use 2nd order instrument_polynomial
+                # to capture both offset and scale, but no frequency response
+                data.remove_response()
+
         if data.count() == 0 and add_empty_channels:
             data += self._get_empty_trace(
                 starttime=starttime,